Field of the Invention
The present invention relates to devices and systems for determining an attitude of an object in a fixed frame of reference by observations from a moving frame of reference and more particularly to a system capable of accurately measuring the verticality of a pile or similar object from a floating platform or vessel. The invention also relates to a method of performing such measurements.
Description of the Related Art
Engineering operations at sea often involve considerable difficulty due to the constant movement of the waves. This is especially the case when accuracy is required in performing measurements. For a vessel at sea, measurements may be made in a fixed frame of reference relative to the earth or in a moving frame of reference relative to the vessel.
One situation where measurements in the absolute frame of reference are relevant is in the installation of fixed structures from a floating platform or vessel. In recent years large numbers of offshore structures have been installed on or in the sea bed, including wind turbines, drilling platforms, bridges, cables, pipelines and the like. The installation work is generally performed from a suitable barge or vessel using cranes and other lifting facilities. In the case of wind turbines, a single pile may be driven into the seabed using a hydraulic hammer or other pile-driving facility mounted on a heavy lift vessel. These monopiles should be installed vertically and it is essential that during driving the verticality is continuously monitored and, where necessary, corrected. At present, the procedure for performing such measurements involves a human operator approaching the monopile and performing a manual measurement of the structure using a handheld inclinometer. This measurement thus takes place directly in the fixed frame of reference of the monopile. During measurement, the pile-driving must be stopped. After evaluation of the results, the crane operator must be informed of the results and instructed with respect to the required corrective action.
In the case of laying of cable from a cable lay vessel, it may be desirable to closely monitor the angle of deployment of the cable in order to determine its trajectory to the seabed. In conventional systems this may be achieved by locating an angle sensor against the cable at the departure location. The sensor may be a pitch and roll type angle sensor which requires direct contact with the cable for operation. Positioning of such sensors can be difficult and they may require frequent attention e.g. on passing joints on the cable. They may also be easily damaged, especially when laying multiple cables and at present are not applicable to all situations. Similar considerations apply during pipe-laying.
It would be desirable to provide a system that simplified such measurements and could be implemented in real time without interrupting operations.